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16.19: Apatite

  • Page ID
    4355
  • Apatite
    Chemical composition Ca5(PO4)3(F,Cl,OH)
    Crystal system Hexagonal
    Habit Prismatic
    Cleavage Imperfect, basal
    Fracture Conchoidal
    Hardness 5
    Optic nature Uniaxial -
    Refractive index 1.63-1.64
    Birefringence 0.003
    Dispersion Low, 0.013
    Specific gravity 3.2
    Lustre Vitreous
    Pleochroism Weak

    File:Apatite.gif
    Figure \(\PageIndex{1}\): Faceted Apatite
    Photo courtesy of The Gem Trader

    File:Apatite2-400.jpg

    Figure \(\PageIndex{2}\): Unusual Violet Faceted Apatite
    Photo courtesy of Jason Brim

    Apatite is named "the deceiver" as it's color, refractometer and specific gravity readings are similar to gemstones as tourmaline, topaz, andalusite and danburite.

    Diagnostics

    Appearance

    Apatite occurs in yellow, green, violet, purple, blue, brown, colorless and pink and can be transparent to translucent.

    Optic Character

    DR, Uniaxial negative

    Refractometer

    Refractometer readings are in the range of tourmaline, topaz, andalusite and danburite but usually with a very low birefringence of 0.003.
    Full range: nω = 1.629 - 1.667 and nε = 1.624 - 1.666 with a full birefringence of 0.001-0.007.

    Spectroscope

    The spectroscope is a good diagnostic tool in separating apatite from gemstones as tourmaline and topaz.

    Blue apatite will, usually, show a rare earth spectrum line in the blue (praseodymium).
    Blue-green stones may have added spectrum lines in the yellow (didymium) and line(s) in the green (praseodymium).
    Yellow gems show the rare earth spectrum of greenish-blue without the line in the blue.
    Neon blue stones may show rare earth spectra in the yellow.

    UV Reactions
    • Blue: Blue to light blue (LW and SW)
    • Yellow: Purplish Pink (stronger in LW)
    • Green: Greenish-yellow (stronger in LW)
    • Violet: Greenish-yellow (LW); Light purple (SW)
    Chelsea Colour Filter

    Green-blue stones: green

    Dichroscope

    Apatite has weak pleochroism, so this tool may not be very useful except for blue stones where the dichroism is more profound (blue and pale yellow).

    Polariscope

    A nice uniaxial negative interference figure should be found.

    Primary Sources

    Brazil, Burma, Mexico
    Violet and Purple are found in Maine and Afghanistan.

    Magnification

    Clouds of tiny white crystal inclusions may be easily observed.

    File:Apatite.jpg

    Figure \(\PageIndex{3}\): Blue apatite with crystalline "fireworks" of an unidentified mineral
    40X Magnification
    by Barbra Voltaire

    File:Apatite accicular.jpg

    Figure \(\PageIndex{4}\): Yellow apatite with a 3-dimensional network of liquid needles
    40X Magnification
    by Barbra Voltaire

    Phenomena

    • Cat's-eyes

    G&G Articles on Apatite 1934-1980

    The GIA has published all the G&G's from 1934 until 1980 online. The organization of the list by subject was done by Joseph Gill.

    • Summer 1962, Cat's-eye apatite, p. 315, 2pp.
    • Summer 1963, Yellow cat's-eye apatite, p. 44, 1p.
    • Winter 1965, Cat's-eye apatite, p. 372, 1p.
    • Summer 1966, A cat's-eye apatite of 220 cts., p. 46, 1p.
    • Winter 1972, Rare Earth Absorption Spectra in Apatite, by Anderson, p. 101, 1p.
    • Winter 1972, A 40 ct. cat's-eye apatite, p. 114, 1p.

    Sources

    • A Students' Guide to Spectroscopy (2003) - Colin Winter FGA, DGA ISBN 0954485300
    • Introduction to Optical Mineralogy 3rd edition (2003), Prof. W.D. Nesse